We have studied 16 healthy volunteers whose lungs were preoxygenated six times each in order to assess the amount of room air entrained during tidal volume preoxygenation if a good seal is not maintained between the face mask and the face. With a fresh gas flow of 10 litre min-1, we found that using gravity alone to hold the face mask in position, more than 20% room air was entrained; if the face mask was held adjacent to the face, more than 40% room air was entrained.